Methods for improving adhesion of aluminum films

ABSTRACT

The described embodiments relate generally to aluminum films and pretreatments for improving the adhesion of aluminum films on substrate surfaces. Methods involve providing three-dimensional adhesion surfaces on the substrate that promote adhesion to a subsequently deposited aluminum film. The methods can avoid the use of strike materials, such as nickel and copper, used in conventional adhesion-promoting treatments. According to some embodiments, methods involve providing an aluminum oxide adhesion layer on the substrate prior to depositing aluminum. According to some embodiments, methods involve providing a zincating layer on the substrate prior to depositing aluminum. According some embodiments, methods involve roughening the surface of the substrate prior to depositing aluminum. Some embodiments involve a combination of two or more substrate pretreatments. Described methods can be used to provide more flexibility in subsequent anodizing processes. In some embodiments, methods involve anodizing the aluminum film and a portion of the substrate.

FIELD

This disclosure relates generally to aluminum films and methods fordepositing aluminum films. In particular, described are various methodsfor improving adhesion of deposited aluminum films.

BACKGROUND

Electroplating is a process widely used in industry to provide a metalcoating having a desirable physical quality on a part. For example,electroplated coatings can provide abrasion and wear resistance,corrosion protection and aesthetic qualities to the surfaces of parts.Electroplated coating may also be used to build up thickness onundersized parts.

Aluminum substrates, in particular, can be difficult to plate sincealuminum surfaces rapidly acquire an oxide layer when exposed to air orwater, and thus tend to inhibit good adhesion of an electrodepositedfilm. In addition, since aluminum is one of the more anodic metals,there is a tendency to form unsatisfactory immersion deposits duringexposure to a plating solution, which can cause discontinuous plating orbreakdown of the plating process. Furthermore, if plating an aluminumfilm, plating methods usually involve the plating of pure aluminum metalonto the substrate. Although pure aluminum has an ordered microstructureand good cosmetic properties, it is relatively soft and easilyscratched. Therefore, there are significant challenges to platingaluminum in industrial applications where durability is a desirablecharacteristic of a plated film.

SUMMARY

This paper describes various embodiments that relate to aluminum filmswith improved adhesion.

According to one embodiment, a method for forming a protective coatingon a surface of an aluminum substrate is described. The method includesforming an adhesion-promoting layer on a surface of the aluminumsubstrate. The adhesion-promoting layer has a number of cavities havingside walls oriented substantially normal to the surface of the aluminumsubstrate. The adhesion-promoting layer is chemically compatible with asubsequent anodizing process. The method also includes depositing analuminum layer on the adhesion-promoting layer. The aluminum layer has anumber of anchor portions disposed within corresponding cavities of theadhesion-promoting layer. The anchor portions engage with the side wallsof the adhesion-promoting layer resisting a shearing force applied tothe aluminum layer securing the aluminum layer to the adhesion-promotinglayer.

According to an additional embodiment, a method for forming an aluminumlayer on a substrate is described. The method includes forming analuminum oxide adhesion layer on the substrate. The aluminum oxideadhesion layer has a number of pores defined by a plurality ofcorresponding pore walls. The method also includes, during the forming,controlling an average pore size of the aluminum oxide adhesion layer bysimultaneously allowing growth of the pore walls and dissolving the porewalls such that the average pore size is sufficiently large to allowaluminum material to form therein during a subsequent aluminum layerdepositing process. The method also includes depositing the aluminumlayer on the aluminum oxide adhesion layer. During the depositing,anchoring portions of the aluminum layer are formed within at least aportion of corresponding pores. The anchor portions engage with the porewalls resisting a shearing force applied to the aluminum layer securingthe aluminum layer to the aluminum oxide layer.

According to a further embodiment, a composite coating for an aluminumsubstrate is described. The composite coating includes a first aluminumoxide layer disposed on the aluminum substrate. The first aluminum oxidelayer has a first hardness. The composite coating also includes a secondaluminum oxide layer disposed on the first aluminum oxide layer. Thesecond aluminum oxide layer being more optically transparent than thefirst aluminum oxide layer. The first aluminum oxide layer is integrallybounded to the second oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIGS. 1A-1C show cross-section views of a part undergoing a pretreatmentinvolving an anodizing process to improve adhesion of a depositedaluminum layer.

FIGS. 2A and 2B show cross-section scanning electron microscope (SEM)views of a part that includes an aluminum oxide adhesion layer formedusing a phosphoric acid anodizing process.

FIGS. 3A-3C show cross-section views of a part undergoing a pretreatmentinvolving a zincating process to improve adhesion of a depositedaluminum layer.

FIGS. 4A-4C show cross-section views of a part undergoing a pretreatmentinvolving a surface roughening process to improve adhesion of adeposited aluminum layer.

FIGS. 5A-5C shows cross-section views of a part undergoing aluminumdepositing and anodizing processes where a portion of substrate isanodized.

FIG. 6 shows a flowchart indicating a high-level process involvingsubstrate pretreatment to improve adhesion of a deposited aluminumlayer.

FIG. 7 shows a plating rack assembly suitable for plating a number ofparts.

DETAILED DESCRIPTION

Representative applications of methods and apparatus according to thepresent application are described in this section. These examples arebeing provided solely to add context and aid in the understanding of thedescribed embodiments. It will thus be apparent to one skilled in theart that the described embodiments may be practiced without some or allof these specific details. In other instances, well known process stepshave not been described in detail in order to avoid unnecessarilyobscuring the described embodiments. Other applications are possible,such that the following examples should not be taken as limiting.

This application relates to aluminum films and providing aluminum filmson substrates. As used herein, the terms “film” and “layer” are usedinterchangeably. Unless otherwise described, as used herein, “aluminum”and “aluminum layer” can refer to any suitable aluminum-containingmaterial, including pure aluminum, aluminum alloys or aluminum mixtures.As used herein, “pure” or “nearly pure” aluminum generally refers toaluminum having a higher percentage of aluminum metal compared toaluminum alloys or other aluminum mixtures. The aluminum films are wellsuited for providing both protective and attractive layers to consumerproducts. For example, methods described herein can be used forproviding protective and cosmetically appealing coatings for enclosuresand casings for electronic devices.

Described herein are methods for improving adhesion of depositedaluminum layers on a substrate. Methods described herein can be used toimprove the adhesion of an aluminum layer to a substrate without the useof a strike layer. Methods involve substrate pretreatments prior todepositing of an aluminum layer. The pretreatments providing athree-dimensional surface having gaps or cavities on the substrate thatcan act as anchoring regions for securing the aluminum layer to thesubstrate. In some embodiments, methods involve providing a thinaluminum oxide adhesion layer on the substrate prior to depositingaluminum. In some embodiments, methods involve providing a zincatinglayer on the substrate prior to depositing aluminum. In someembodiments, methods involve roughening the surface of the substrateprior to depositing aluminum. Some embodiments involve a combination oftwo or more substrate pretreatments.

These and other embodiments are discussed below with reference to FIGS.1-7. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these Figures is forexplanatory purposes only and should not be construed as limiting.

As described above, it can be difficult to deposit onto aluminumsubstrates since aluminum substrates quickly acquire a natural oxidelayer when exposed to air or water. The natural oxide layer can inhibitthe adhesion of many metal materials, such as aluminum, to the surfaceof the aluminum substrate. Conventional methods for providing betteradhesion include forming a thin layer of copper or nickel plating,referred to as a strike or strike layer. The strike layer generally hasgood adhesion to the aluminum substrate and also to the subsequentlydeposited aluminum layer. However, use of a strike layer can have hasseveral disadvantages. For example, a strike layer can make the partmore susceptible to galvanic corrosion during a plating process. Inparticular, if external coating layers are scratched exposing the strikelayer next to the plated aluminum layer (and possibly the aluminumsubstrate), the exposure of dissimilar materials can create a galvaniccell on the part. This can increase the risk of corrosion in the platedand anodized part later on.

There can also be manufacturing challenges to using a strike layer. Insome manufacturing processes, an entire aluminum layer is converted toaluminum oxide using an anodizing process. During the anodizing process,the strike layer can be exposed in localized areas creating a variedcurrent density distribution across the part. Locally thinner areas ofthe aluminum layer can become anodized through sooner, resulting in ananodized layer having a varying thickness across the part. Furthermore,materials from the strike layer can contaminate the anodizing bath andcreate defects in the resultant aluminum oxide. To avoid exposing thestrike layer during these processes, a buffer layer of plated aluminumcan be positioned between the substrate and the aluminum oxide layer.However, the buffer layer can add thickness to the overall aluminum andaluminum oxide stack.

To avoid reaching the strike layer, a buffer layer of aluminum can beleft between the strike layer and the remainder of the aluminum layer.Since the thickness of the aluminum layer can be variable across a part(due to variations in current density), the thickness of the bufferlayer is generally dictated by the minimum thickness across the part.One of the disadvantages of using a buffer layer is that it can add anundesired extra thickness to the aluminum and aluminum oxide stack. Inaddition, if during an anodizing process the aluminum oxide layer growstoo close to or beyond the thickness of the aluminum layer, theanodizing solution can contact and react with the strike layer. Thereaction products can contaminate the anodizing solution and causedefects in the resultant aluminum oxide layer. For at least thesereasons, it can be beneficial in certain applications to avoid the useof a strike layer. However, it can be difficult to plate aluminumdirectly onto substrates since aluminum generally does not adhere wellto substrates during electroplating, especially when plating pure ornearly pure aluminum. In addition, if the substrate is also made ofaluminum, the aluminum substrate has a strong affinity to form a naturaloxide layer on its surface making it difficult to plate thereon.

In order to improve the adhesion of an aluminum layer on a substrate,methods described herein involve pretreating the substrate prior to analuminum deposition process. The pretreatments avoid the use of a strikelayer and, therefore, do not include some of the downsides of using astrike layer. The pretreatments involve creating a three-dimensionaladhesion-promoting surface on the substrate. When an aluminum layer isdeposited on the adhesion-promoting surface, portions of the aluminumcan become deposited within the gaps or cavities of thethree-dimensional adhesion-promoting surface. These portions of thealuminum layer can anchor the aluminum layer to the substrate surfaceand provide better adhesion of the aluminum layer to the substrate. Theadhesion-promoting layer can be substantially free of non-aluminum metalagents, such as agents containing copper and/or nickel, and thereforechemically compatible with a subsequent anodizing process. In someembodiments, creating the adhesion-promoting surface involves forming analuminum oxide adhesion layer on the substrate surface, which isdescribed below with reference to FIGS. 1A-1C and 2A-2B. In otherembodiments, creating the adhesion-promoting surface involves forming azincating layer on the substrate surface, which is described below withreference to FIGS. 3A-3C. In other embodiments, creating theadhesion-promoting surface involves roughening the substrate surface,which is described below with reference to FIGS. 4A-4C. It should beunderstood that these embodiments are presented as suitable examples andare not meant to limit the types and scope of possible methods ofproviding adhesion-promoting surfaces.

One way of forming an adhesion-promoting surface on a substrate is byforming a thin aluminum oxide layer that has adhesion-promotingproperties on the substrate. FIGS. 1A-1C show cross-section views ofpart 100 undergoing an aluminum deposition process involving formationof an aluminum oxide adhesion layer in accordance with some embodiments.At FIG. 1A, a thin portion of aluminum substrate 102 is converted toaluminum oxide adhesion layer 104. Prior to forming aluminum oxideadhesion layer 104, the top surface of aluminum substrate 102 can betreated with any suitable finishing technique. For example, aluminumsubstrate 102 can be polished to have mirror shine. In otherembodiments, aluminum substrate 102 is textured to have a textured orroughened surface. Since forming aluminum oxide adhesion layer 104 is aconversion process whereby a portion of aluminum substrate 102 isconsumed, aluminum oxide adhesion layer 104 is integral to and welladhered to aluminum substrate 120. Aluminum oxide adhesion layer 104should be thick enough to create a good anchoring surface and thinenough so that the surface of substrate 102 remains conductive for asubsequent electroplating process. In some embodiments, aluminum oxideadhesion layer 104 has a thickness of less than about 5 micrometers. Insome embodiment, aluminum oxide adhesion layer 104 has a thickness ofabout 3 micrometers or less.

Aluminum oxide adhesion layer 104 can be formed using an anodizingprocess that includes the use of an acidic electrolytic solution. Insome embodiments, the electrolytic solution includes phosphoric acid,oxalic acid, or a combination of phosphoric acid and oxalic acid. Thephosphoric and/or oxalic acid can promote the formation of pores 105having a larger average diameter compared to the average diameter ofstandard anodic pores. In addition, aluminum oxide adhesion layer 104generally has lower pore density compared to standard anodized aluminumoxide layers. It is believed that the phosphoric and/or oxalic acidtends to dissolve portions of the pore walls of pores 105 as pores 105are being grown, thereby creating the larger diameter pores 105 andlower pore density. That is, anodizing in phosphoric acid and/or oxalicacid conditions allow for simultaneous growth and dissolving of the porewalls. In some embodiments, pores 105 have a diameter of about 100 nm orgreater. In some embodiments, the acidic electrolytic solution containschromic acid and/or sulfuric acid. In some embodiments, the acidicelectrolytic solution contains a mixture of two or more of phosphoricacid, oxalic acid, chromic acid, and sulfuric acid. In some embodiments,aluminum oxide adhesion layer 104 is exposed to an inert atmosphereprior to a subsequent aluminum plating process in order to activatealuminum oxide layer 104 and promote better adhesion with the platedaluminum. The inert atmosphere can include exposing part 100 to anon-oxidative atmosphere such as a nitrogen or argon environment.

At FIG. 1B, aluminum layer 106 is deposited on aluminum oxide adhesionlayer 104. Aluminum layer 106 can be deposited using any suitableprocess including a plating process, such as an electroplating process.In some embodiments, aluminum layer 106 is deposited on aluminum oxideadhesion layer 104 very soon after formation of aluminum oxide adhesionlayer 104 to avoid exposure of aluminum oxide adhesion layer 104 tomoisture or air. In some embodiments, aluminum oxide adhesion layer 104is placed in a moisture free atmosphere, such as a nitrogen or argonenvironment. As shown, pores 105 are sufficiently large such thatanchoring portions 107 of aluminum layer 106 can form within pores 105.Pore walls 105 are substantially normal to the top surface of substrate102 such that when a shearing force 109 is applied to aluminum layer106, anchoring portions 107 can engage with pore walls 105 and securealuminum layer 106 to aluminum oxide adhesion layer 104. The adhesionstrength of aluminum layer 106 to aluminum oxide adhesion layer 104 canbe measured by a standard pull test. In some embodiments, the adhesionof aluminum layer 106 to aluminum oxide adhesion layer 104, as measuredby pull testing, ranges from about 70 MPa and about 85 MPa.

At FIG. 1C, a portion of aluminum layer 106 is optionally converted toaluminum oxide layer 108. As shown, aluminum oxide layer 108 has pores112 that are generally smaller in diameter compared to pores 105 ofaluminum oxide adhesion layer 104. Aluminum oxide layer 108 can beformed using any suitable process, including conventional anodizingprocesses. Note that part 100 does not include a strike layer, therebyeliminating the occurrence of defects caused by anodizing interferingmaterials in a strike layer. That is, there is no chance of reaching adefect-causing material during an anodizing process. Thus, aluminumlayer 106 can be thinner than a corresponding aluminum layer using astrike layer, thereby reducing the stack thickness of aluminum oxideadhesion layer 104, aluminum layer 106, and aluminum oxide layer 108. Insome embodiments, substantially all of aluminum layer 106 is convertedto an aluminum oxide layer. In some embodiments, substantially all ofaluminum layer 106 and a portion of aluminum substrate 102 are convertedto an aluminum oxide layer. These embodiments are achievable becausethere is no intervening strike layer. That is, aluminum oxide adhesionlayer 104 is compatible with an anodizing process.

FIGS. 2A and 2B shows cross-section SEM (scanning electron microscope)images of an actual aluminum part 200 illustrating the featuresdescribed above with reference to FIGS. 1A-1C. FIG. 2B is an insetshowing a close-up view of a portion of the SEM image of FIG. 2A. Part200 includes aluminum substrate 202, aluminum oxide adhesion layer 204,aluminum layer 206, and aluminum oxide layer 208. Aluminum oxideadhesion layer 204 was formed using an anodizing solution containingphosphoric acid. As clearly shown in the inset FIG. 2B, portions ofaluminum layer 206 deposit within the pores of aluminum oxide adhesionlayer 204, mechanically interlocking aluminum layer 206 and aluminumoxide adhesion layer 204 together. The pores of aluminum oxide adhesionlayer 204 are substantially normal to the top surface of substrate 202such that when a shearing force is applied to aluminum layer 206, thedeposited portions of aluminum layer 206 can engage with the pore wallsof the pores and secure aluminum layer 206 to aluminum oxide adhesionlayer 204.

Another way forming an adhesion-promoting surface on a substrate is byforming a zincating layer on the substrate. FIGS. 3A-3C showcross-section views of part 300 undergoing an aluminum depositionprocess involving formation of a zincating layer in accordance with someembodiments. At FIG. 3A, zincating layer 304 is deposited onto aluminumsubstrate 302. Zincating layer 304 is generally a thin conductivecrystalline layer that can be formed by exposing aluminum substrate 302to a zincate solution. The zincate solution containstetrahydroxozincating ion (Zn(OH)₄ ²), which can remove a natural oxidelayer that forms on aluminum substrate 302. Once formed, the zincatinglayer 304 can prevent re-oxidation of aluminum substrate 302. In someembodiments, a cyanide multi-metal based zincatc solution is used.Zincating layer 304 is generally chemically computable with a subsequentanodizing process. In some embodiments, zincating layer 304 is verythin, in some cases less than about 0.5 micrometers thick. In someembodiments, zincating layer 304 is exposed to an inert atmosphere priorto a subsequent aluminum plating process in order to promote betteradhesion with the plated aluminum.

At FIG. 3B, aluminum layer 306 is deposited on zincating layer 304. Asshown by the inset view, zincating layer 304 has a three-dimensionalcrystalline structure that includes cavities surrounded by walls 305.When aluminum layer 306 is deposited onto zincating layer 304, anchoringportions 307 become deposited within the cavities of zincating layer304. Walls 305 can be substantially normal to the top surface ofsubstrate 302 such that when a shearing force 309 is applied to aluminumlayer 306, anchoring portions 307 can engage with walls 305 and securealuminum layer 306 to substrate 302. In some embodiments, the adhesionof aluminum layer 306 to zincating layer 304, as measured by pulltesting, ranges from about 30 MPa and about 65 MPa.

At FIG. 3C, a portion of aluminum layer 306 is optionally converted toaluminum oxide layer 308, which has anodic pores 312 formed therein.Aluminum oxide layer 308 can be formed using any suitable process,including a conventional anodizing process. Zincating layer 304 allowsfor the elimination of a strike layer within part 300, therebyeliminating the occurrence of defects caused by anodizing interferingmaterials in a strike layer. Thus, in some embodiments, substantiallyall of aluminum layer 306 is converted to an aluminum oxide layer. Insome embodiments, substantially all of aluminum layer 306 and a portionof aluminum substrate 302 are converted to an aluminum oxide layer.

An additional way of forming an adhesion-promoting surface on asubstrate is by creating a textured or roughened surface on thesubstrate. FIGS. 4A-4C show a cross-section view of part 400 undergoingan aluminum deposition process involving substrate surface roughening inaccordance with some embodiments. At FIG. 4A, top surface of aluminumsubstrate 402 is textured to have a rough surface 404 having a series ofpeaks and valleys. Any suitable surface texturing or roughing processcan be used. In some embodiments, a blasting procedure is used, wherebya blasting media is impinged on top surface of aluminum substrate 402.In some embodiments, a laser texturing procedure is used, whereby acontinuous or pulsed laser is scanned across the top surface of aluminumsubstrate 402 creating random or organized patterns of pits. In someembodiments, an acid etching procedure is used, whereby an acidicsolution etches and creates a roughened top surface of aluminumsubstrate 402. In some embodiments, aluminum substrate 402 is exposed toan inert atmosphere prior to a subsequent aluminum plating process inorder to promote better adhesion with the plated aluminum.

At FIG. 4B, aluminum layer 406 is deposited on aluminum substrate 402.As shown, anchoring portions 407 are formed within the valleys ofroughened surface 404. Walls 405 of the valleys of roughened surface 404are substantially normal to the top surface of substrate 402 such thatwhen a shearing force 1309 is applied to aluminum layer 406, anchoringportions 407 engage walls 405 securing aluminum layer 406 to substrate402. Note that each of the valleys of rough surface 404 are generallylarger in width compared to the width of each pore 105 of aluminum oxideadhesion layer 104 described above with reference to FIGS. 1A-1C. Insome embodiments, the adhesion of aluminum layer 306 to substrate 402,as measured by pull testing, is about 29 MPa.

At FIG. 4C, a portion of aluminum layer 406 is optionally converted toaluminum oxide layer 408, which has pores 412 formed therein. Aluminumoxide layer 408 can be formed using any suitable process, including aconventional anodizing process. In some embodiments, substantially allof aluminum layer 406 is converted to an aluminum oxide layer. Roughenedsurface allows for the elimination of a strike layer within part 400,thereby eliminating the occurrence of defects caused by anodizinginterfering materials in a strike layer. Thus, in some embodiments,substantially all of aluminum layer 406 is converted to an aluminumoxide layer. In some embodiments, substantially all of aluminum layer406 and a portion of aluminum substrate 402 are converted to an aluminumoxide layer.

In some embodiments, one or more of the above-described pretreatmenttechniques can be used in combination. For example, an aluminumsubstrate can be treated with a surface roughening process, followed byformation of an aluminum oxide adhesion layer, and followed bydeposition of an aluminum layer. In another embodiment, an aluminumsubstrate is treated with a surface roughening process, followed byformation of a zincating layer, and followed by deposition of analuminum layer. In some embodiments, combining multiple pretreatmenttechniques can improve the adhesion of an aluminum layer to a substrate.

As described above, one of the advantages of the absence of a strikelayer is that more portions of the aluminum layer, and possibly thesubstrate itself, can be converted to aluminum oxide without creatingstrike layer material defects. This allows for more flexibility during asubsequent anodizing process and the more possible variations in formingaluminum oxide layers on a substrate. FIGS. 5A-5C shows cross-sectionviews of part 500 undergoing aluminum depositing and anodizing processeswhere a portion of substrate is anodized.

At FIG. 5A, adhesion-promoting surface 504 is formed on aluminumsubstrate 502. Adhesion-promoting surface 504 is any suitable surfacethat has a three-dimensional quality that allows for formation ofanchoring portions during a subsequently aluminum depositing process. Asdescribed above, adhesion-promoting surface 504 can correspond to asurface of an aluminum oxide adhesion layer, a surface of a zincatinglayer, or a roughed surface of substrate 502. Substrate 502 can be madeof any suitable anodizable metal or metal alloy.

At FIG. 5B, aluminum layer 506 is deposited on adhesion-promoting layer.Aluminum layer 506 can have the same or different composition assubstrate 502. In one embodiment, aluminum layer 506 and substrate 502are both made of the substantially the same aluminum alloy. In someembodiments, aluminum layer 506 is made of substantially pure aluminumand substrate 502 is made of an aluminum alloy. Embodiments wherealuminum layer 506 is substantially pure aluminum may be preferable inapplications where it is desirable to have an optically brighter toplayer for part 500. Substrate 502 can be made of aluminum alloy sincealuminum alloy is generally harder than pure aluminum and can providegood structural support for part 500.

At FIG. 5C, substantially all of aluminum layer 506 is converted tofirst oxide layer 508 and a portion of substrate 502 is converted tosecond oxide layer 510, forming a composite coating for part 500. Sincefirst oxide layer 508 and second oxide layer 510 are formed during asingle anodizing process, first oxide layer 508 can be integrally bondedwith second oxide layer 510. First oxide layer 508 and second oxidelayer 510 are separated by interface 514. Pores 512 formed during theanodizing process can be formed within first oxide layer 508, transcendinterface 514 and continue to within second oxide layer 510. Inembodiments where aluminum layer 506 and substrate 502 have differentcompositions, first oxide layer 508 and second oxide layer 510 can havedifferent physical qualities and/or appearances. For example, analuminum oxide layer resulting from conversion of a pure aluminum layercan be more optically transparent or translucent compared to an aluminumoxide layer resulting from conversion of an aluminum alloy layer. Thatis, aluminum oxide obtained from aluminum alloys can appear more yellowand have more of a hazy or matt quality. First oxide layer 508 andsecond oxide layer 510 can also have different hardness qualities. Inone embodiment, second oxide layer 508 is harder than first oxide layer510.

FIG. 6 shows flowchart 600 indicating a high-level process involvingsubstrate pretreatment and aluminum depositing, in accordance withdescribed embodiments. At 602, a surface of a substrate is pretreatedforming an adherence-promoting surface. The substrate can be made of ananodizable material such as aluminum or alloys thereof. Thepretreatments can include providing a three-dimensional surface that hasgaps or cavities. Examples of pretreatments include one or more offorming an aluminum oxide adherence layer, forming a zincating layer andproviding a roughen substrate surface. In some embodiments, theadhesion-promoting layer has a thickness of less than about 3micrometers. At 604, the adherence-promoting surface is optionallyactivated by exposing the adherence-promoting surface to an inertatmosphere. Suitable inert atmospheres can include exposure to nitrogenand/or argon gas.

At 606, an aluminum layer is deposited onto the adherence-promotingsurface of the substrate. In some embodiments, the aluminum layer isdeposited using a plating process, such as an electroplating process.The aluminum layer can have substantially the same or differentcomposition as the substrate. In one embodiment, the substrate is madeof an aluminum alloy and the aluminum layer is made of substantiallypure aluminum. The aluminum layer can be deposited to any suitablethickness. In some embodiments, the aluminum layer is deposited to athickness ranging from about 1 micrometer and about 10 micrometers. Insome embodiments, the aluminum layer is deposited to a thickness rangingfrom about 2 micrometers and about 4 micrometers.

At 608, at least a portion of the aluminum layer of the aluminum layerand the substrate is optionally converted to an oxide layer. In someembodiments, an anodizing process is used to form the oxide layer. Insome embodiments, only a portion of the aluminum layer is converted toan aluminum oxide layer. The absence of a strike layer makes it possibleto allow the anodizing process to convert a relatively larger percentageof the aluminum layer without concern as to strike layer materialrelated defects. Thus, in some embodiments, substantially the entirealuminum layer, including portions proximate the substrate, is convertedto aluminum oxide. In some embodiments, substantially the entirealuminum layer is converted to an aluminum oxide layer and a portion ofthe substrate is converted to an oxide layer. Anodizing processconditions can be chosen such that the aluminum oxide layer is durableand cosmetically appealing. In general, an aluminum oxide layerconverted from a substantially pure aluminum layer can have a relativelytransparent or translucent visual quality compared to an aluminum oxidelayer converted from an aluminum alloy.

In a production environment, a number of parts can be plating in asingle plating bath. The parts can be situated on a rack assembly, suchas rack assembly 700 shown in FIG. 7. Rack assembly 700 is configured tosupport parts 702 a-7021 during a plating process and, in someembodiments, during processes prior to or subsequent to a platingprocess. For example, rack assembly 700 can be used to support parts 702a-7021 during forming of an adhesion-promoting surface, during exposureto an inert atmosphere, during a plating process and/or during apost-plating anodizing process. This way, parts 702 a-7021 can betransferred together as a unit from process station to process stationwithout removing parts 702 a-7021 from rack assembly 700.

Rack assembly 700 can be placed within a plating bath during a platingprocess with bottom portion 711 oriented toward a bottom of the platingcell and top portion 713 oriented toward a top of the plating cell. Rackassembly 700 includes rack frame 704, drainage bars 706, and cut outs710. Parts 702 a-7021 can be positioned within cut outs 710 such thateach of parts 702 a-7021 is separated a distance 712 from an edge ofrack frame 704. In addition, outward surfaces of parts 702 a-7021 andoutward surfaces of rack frame 704 are along the same plane. Distance712 should be small enough such that, during a plating process, parts702 a-7021 and rack frame 704 approximate a single flat surface. Theproximity of parts 702 a-7021 to rack frame 704 and the positioning ofparts 702 a-7021 along the same plane as rack frame 704 can promote evencurrent density and plating along edges, corners, and flat surfaces ofparts 702 a-7021. In some embodiments, drainage bars 706 are added torack assembly 700. Drainage bars 706 are connected with and extendoutward from rack frame 704 along a different plane as parts 702 a-7021and rack frame 704. Drainage bars 706 can be positioned at an anglerelative to rack frame 704 to promote good drainage of chemicals duringthe plating process. Drainage bars 706 can include connector portions708 that connect with and fix parts 702 a-7021 to drainage bars 706. Insome embodiments, connector portions 708 are secured to parts 702 a-7021using fasteners such as screws. It should be noted that rack assembly700 illustrates a particular embodiment and that the shape andarrangement of rack frame 704, drainage bars 706 and parts 702 a-7021can vary in other embodiments.

It should be noted that in processing aluminum alloy substrates forcoating with a high purity aluminum, the rack material should bechemically compatible with various processing steps that may beemployed. In some embodiments, the adhesion improvement processing(e.g., phosphoric anodizing) requires that substantially all surfacespresented for processing uniformly evolve a tenacious dielectric oxidelayer for the process to proceed correctly. A subsequent processing step(inert atmosphere activation) can also benefit from having only aluminumsurfaces exposed. Bare titanium can work successfully for adhesion butpotentially cause cosmetic defects. Use of aluminum coated titaniumracks avoids these defects.

Racks made entirely of an aluminum alloy may be successfully employedfor the adhesion improvement step, the inert activation step and alsofor any cosmetic finish anodization after the high purity aluminumcoating process, without changing the rack. In some embodiments, atitanium rack may also be employed for all these processing steps if itis first coated with aluminum. The utility of the titanium rack is thatit will not be substantially degraded by normal post processing cleaningand restoration treatments. A rack made entirely of aluminum couldpotentially be consumed and destroyed by some number of completeprocessing cycles.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. Thus, the foregoing descriptions of specific embodimentsare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the described embodiments to theprecise forms disclosed. It will be apparent to one of ordinary skill inthe art that many modifications and variations are possible in view ofthe above teachings.

1. A method of forming a protective coating on a surface of an aluminumsubstrate, the method comprising: forming an adhesion-promoting layer ona surface of the aluminum substrate, the adhesion-promoting layer havinga plurality of cavities having side walls oriented substantially normalto the surface of the aluminum substrate, wherein the adhesion-promotinglayer is chemically compatible with a subsequent anodizing process; anddepositing an aluminum layer on the adhesion-promoting layer, thealuminum layer having a plurality of anchor portions disposed withincorresponding cavities of the adhesion-promoting layer, wherein theanchor portions engage with the side walls of the adhesion-promotinglayer resisting a shearing force applied to the aluminum layer securingthe aluminum layer to the adhesion-promoting layer.
 2. The method ofclaim 1, further comprising: converting at least a portion of thealuminum layer to an aluminum oxide layer using an anodizing process. 3.The method of claim 2, wherein substantially the entire aluminum layeris converted to the aluminum oxide layer.
 4. The method of claim 3,further comprising: during the anodizing, transcending a boundarybetween the adhesion-promoting layer and the aluminum substrateconverting a portion of the aluminum substrate to a second aluminumoxide layer.
 5. The method of claim 1, wherein the aluminum substratemakes up at least a portion of an enclosure for an electronic device. 6.The method of claim 1, wherein the adhesion-promoting layer issubstantially free of copper or nickel.
 7. The method of claim 1,wherein the aluminum oxide adhesion layer has a thickness of about 3micrometers or less.
 8. The method of claim 1, wherein forming theadhesion-promoting layer comprises: forming an aluminum oxide adhesionlayer on the surface of the aluminum substrate, the aluminum oxideadhesion layer having a plurality of pores defined by a plurality ofcorresponding pore walls, wherein the plurality of pores have an averagepore size sufficiently large to allow anchoring portions of the aluminumlayer to form therein during the subsequent aluminum layer depositingprocess.
 9. The method of claim 8, wherein the aluminum oxide adhesionlayer has a thickness of about 3 micrometers or less.
 10. The method ofclaim 1, wherein forming the adhesion-promoting layer comprises: forminga zincate layer on the aluminum substrate, the zincate layer having acrystalline structure having the plurality of cavities.
 11. The methodof claim 10, wherein the zincate layer has a thickness of less thanabout 0.5 micrometers.
 12. The method of claim 1, further comprising:prior to forming the adhesion-promoting layer, roughening the surface ofthe aluminum substrate.
 13. The method of claim 12, wherein the aluminumsubstrate is comprised of an aluminum alloy.
 14. The method of claim 1,wherein the aluminum substrate is comprised of an aluminum alloy.
 15. Amethod for forming an aluminum layer on a substrate, the methodcomprising: forming an aluminum oxide adhesion layer on the substrate,the aluminum oxide adhesion layer having a plurality of pores defined bya plurality of corresponding pore walls; during the forming, controllingan average pore size of the aluminum oxide adhesion layer bysimultaneously allowing growth of the pore walls and dissolving the porewalls and dissolving that the average pore size is sufficiently large toallow aluminum material to form therein during a subsequent aluminumlayer depositing process; and depositing the aluminum layer on thealuminum oxide adhesion layer, wherein during the depositing, anchoringportions of the aluminum layer form within at least a portion ofcorresponding pores, wherein the anchor portions engage with the porewalls resisting a shearing force applied to the aluminum layer andsecuring the aluminum layer to the aluminum oxide layer.
 16. The methodof claim 15, wherein the substrate is comprised of an anodizablematerial, the method further comprising: converting substantially theentire aluminum layer to an aluminum oxide layer and converting aportion of the substrate to an oxide layer.
 17. The method of claim 15,wherein the substrate is comprised of an aluminum alloy.
 18. The methodof claim 16, wherein the substrate is comprised of an aluminum alloy.19. The method of claim 15, wherein the aluminum layer is comprise ofsubstantially pure aluminum.
 20. The method of claim 15, wherein thealuminum substrate makes up at least part of an enclosure for anelectronic device.
 21. (canceled)
 22. (canceled)
 23. (canceled) 24.(canceled)
 25. (canceled)
 26. (canceled)
 27. (canceled)
 28. (canceled)29. (canceled)
 30. (canceled)